Posted
by
Unknown Lamer
on Tuesday September 04, 2012 @04:13AM
from the 1950s-science-strikes-back dept.

DevotedSkeptic writes with an interesting article on possible lighting sources for growing food on the moon and other off-world locations. From the article: "... Agriculture remains the key to living and working off-world. All the mineral ore in the solar system can't replace the fact that for extended periods on the Moon or Mars, future off-worlders will need bio-regenerative systems in order to prosper. Here on earth, researchers still debate how best to make those possible, but nuclear-powered state of the art LED technology is arguably what will drive photosynthesis so necessary to provide both food and oxygen for future lunar colonists. ... Although during the two weeks that make up the long lunar day astronauts might be able to funnel refracted sunlight into covered greenhouses or subsurface lava tunnels, they will be left without a light source during the long lunar night. Current solar-powered battery storage technology isn't adequate to sustain artificial light sources for two weeks at the time. Thus, the most practical solution is simply to use some sort of Radioisotope Thermoelectric Generator, not unlike the one powering the current Mars Science lab, to power the LEDs that will spur photosynthesis in lunar greenhouses. ... On earth, Mitchell says it takes roughly 50 square meters of agriculture to provide both food and oxygen life to support one human. But, as he points out, who can say how productive plants are ultimately going to be on the moon, in gravity that is only one sixth that of earth?"

You've touched on a very important point. There has been a lot of work on this topic. For small diameters the spin rate is so high that the coriolis force on your body is disruptive - your feet want to go one way, your head in another. The centrigugal force is also different at the feet and the head. And the stars would be going by outside the window (if there are any) at rather startling rates.

So 200-300 meters becomes the most reasonable minimum radius. According to NASA via Wikipedia [wikipedia.org]:

Turning one's head rapidly in such an environment causes a "tilt" to be sensed as one's inner ears move at different rotational rates. Centrifuge studies show that people get motion-sick in habitats with a rotational radius of less than 100 metres, or with a rotation rate above 3 rotations per minute. However, the same studies and statistical inference indicate that almost all people should be able to live comfortably in habitats with a rotational radius larger than 500 meters and below 1 RPM. Experienced persons were not merely more resistant to motion sickness, but could also use the effect to determine "spinward" and "antispinward" directions in the centrifuges.

Fortunately, for a reasonably large structure, the additional strength required to support such a rotation rate is not large compared to the strength required to support normal atmospheric pressure and other requirements. So it's not a deal-breaker.More detail here [wikipedia.org], and here (O'Neill colonies) [wikipedia.org]. O'Neill proposed cylinders eight km in diameter and 32 km long, with a population of (IIRC) 20,000, built with materials from the moon.

B. "Now LED's are at about 100 lumens per watt" - WRONG AGAIN. We have 5500K white LEDs with 150+ lumens per watt, and Cree has already broken 220+ lux/w - LAST YEAR.

C. "So you need about 1,300 watts to light up one square meter to the same intensity as sunlight. Very roughly." Sure, but you're implying most of our food crops even need that sort of intensity - they don't.

D. "Solar cells and inverters and wiring have an end-to-end efficiency of around 10%" Yea, if you use cheapo garbage. The stuff powering my research facility, end-to-end, pushes roughly 22%.

E. "So we need about 13 meter-square panels at right-angles all the time to the Sun to get 13,000 watts during sunny days on the Moon." I see you totally ignore the fact that our moon has no atmosphere worth mentioning, so that photon flux density is actually much higher versus on earth, you also forget that the moon is closer to the sun then we are roughly half of the time, so again, the photon flux is even greater.

G. "And oh, where are you going to get the water for 50 square meters of whatnot growing?" Plenty of hydrogen and oxygen on the moon, plus we've found water there. We can make fake snow by just expelling compressed hydrogen and oxygen in a shared jet nozzle (it's how we make snow during the summer on mountain ski resorts) so I bet making water from scratch components would not be that difficult. On top of that, we've got hydroponics systems that can drop water requirements as much as 99% for many crops.

Your numbers fail to take into account how plants grow and just how much space is needed.

And as an aside - I [tinypic.com] do [tinypic.com] this [imgur.com] professionally. [imgur.com] I'm going to have to say your words are sorely lacking in knowledge on the relevant subjects.